In analyzing the timing of an engine, the engine speed and timing angle must be determined. The timing angle is the number of degrees of revolution of the crankshaft angle between top-dead-center of a selected cylinder and combustion in that cylinder. Automobile manufacturers commonly specify the number of degrees at a specific engine speed as a compromise to numerous considerations which must be taken into account.
Top dead center is identified in the following manner. The engine includes a wheel mounted on the end of the engine crankshaft so as to rotate therewith. A notch or other marking means is located in the periphery of the wheel. The receptacle for a magnetic probe is positioned on the engine block and is so located that the notch on the rotating wheel will pass the receptacle a known number of degrees of crankshaft rotation after the number one, or other selected cylinder, has reached its top-dead-center position.
The combustion event may be sensed in a number of ways. For example, in a spark-ignited engine a clip is normally applied to the conductor associated with a selected spark plug which produces pulses corresponding to the combustion event. In a diesel engine, the combustion event could be sensed by light occurring during combustion. The glow plug for the prechamber associated with the selected cylinder is removed and a luminosity probe is inserted in its place. The luminosity probe includes a sensor which responds to light produced during combustion and generates an electrical signal.
It is important that the readings furnished by the timing meter be accurate. For example, if the meter indicates that the engine speed is 1,200 rpm and/or that the timing angle is 20.degree., such readings must be correct. A calibrator is used to provide signals of known and appropriate values to the meter so that the displayed values can be checked for function and accuracy.